Synthesis and investigation of the morphological, optical, electrical and dielectric characteristics of a disodium cobalt orthogermanate.

A sodium cobalt germanate material (Na2CoGeO4) was synthesized and studied. The X-ray powder diffraction pattern revealed a monoclinic crystal system with the Pn space group. The calculated value of the direct bandgap (Eg) was (3.89 ± 0.02) eV. Electrical measurements were undertaken in a frequency range from 40 Hz to 1 MHz and a temperature range from 403 K to 573 K. Nyquist plots revealed only a semicircle in the whole impedance spectra due to the effect of interior grains. The AC conductivity of Na2CoGeO4 was associated with the correlated barrier hopping (CBH) model. Studies of the complex electric modulus M*(ω) confirmed that the relaxation process was thermally activated.

Synthesis, optical and ionic conductivity studies of a lithium cobalt germanate compound.

A lithium cobalt germanate compound (Li2CoGeO4) was synthesized and studied. The X-ray powder diffraction pattern demonstrated a monoclinic crystal system with the Pn space group. The morphology and composition were done by scanning transmission electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS). A vibrational study confirmed the existence of the anion (GeO4)4- and its vibrations. The estimated value of the direct band gap (E g) was evaluated at 3.45 eV. The measurements of the electrical properties were performed in the frequency interval from 100 Hz to 1 MHz and the temperature range from 553 K to 663 K. The Nyquist plots revealed the existence of a single semicircle in all impedance spectra due to the grain interior effect. The AC electrical conduction in Li2CoGeO4 has been explained through several processes, which can be coupled with two different formalisms. The complex electric modulus studies M*(ω) confirmed that the relaxation process is thermally activated.

Syntheses and Characterizations of CuIn1-xZnxSe2 Chalcopyrite Nanoparticles.

CuIn1-xZnxSe2 powders with various atomic percentages (x = 0, 0.05, 0.11, 0.16 and 0.21) were synthesized with the solvothermal method using metal chlorides and ethylendiamine as sources of precursors and a solvent, respectively. The experiment aims to investigate the effect of atomic percentages of Znx compounds on the structural and optical properties of CuIn1-xZnxSe2 in order to improve future technological applications based on this material. The powders' chalcopyrite phases were identified by X-ray diffraction. Energy dispersive X-ray spectroscopy analysis revealed the presence of Cu, In, Zn and Se with the expected atomic ratio of Zn/(In + Zn). Scanning electron microscopy and transmission electron microscopy analysis showed that the powders have large-scale desert rose-like structures. The nanopowders' optical study by UV-visible spectrophotometry showed that the CuIn1-xZnxSe2 energy gap values increase with the molar fraction of Znx. A change from 1.15 to 1.4 eV was observed.

Single-Particle Tracking with Scanning Non-Linear Microscopy.

This study describes the adaptation of non-linear microscopy for single-particle tracking (SPT), a method commonly used in biology with single-photon fluorescence. Imaging moving objects with non-linear microscopy raises difficulties due to the scanning process of the acquisitions. The interest of the study is based on the balance between all the experimental parameters (objective, resolution, frame rate) which need to be optimized to record long trajectories with the best accuracy and frame rate. To evaluate the performance of the setup for SPT, several basic estimation methods are used and adapted to the new detection process. The covariance-based estimator (CVE) seems to be the best way to evaluate the diffusion coefficient from trajectories using the specific factors of motion blur and localization error.

Fano-resonant silicon photonic crystal slab for efficient third-harmonic generation.

Strong light-matter interactions in resonant photonic nanostructures open up opportunities for enhancing nonlinear responses. In this work, by applying Fano resonances, we experimentally demonstrate efficient third-harmonic generation (THG) obtained with 2D silicon photonic crystal slabs (PCSs) thanks to the field enhancement in the dielectric layer. A 160-fold enhancement of THG is observed in the silicon PCS compared to the unpatterned silicon film. Through slightly changing the radius of the PCS, tunable THG on a single chip is obtained, paving a way for the optical manipulation of harmonic generation.

Enhanced linear absorption coefficient of in-plane monolayer graphene on a silicon microring resonator.

We demonstrate that enhanced linear absorption coefficient (LAC) of in-plane monolayer graphene is determined by the optical transmission spectra of a graphene layer coated symmetrically coupled add-drop silicon microring resonator (SC-ADSMR), of which the value is around 0.23 dB/µm. In contrast to the traditional cut-back method, the measured results aren't dependent on the coupling efficiency between the fiber tip and the waveguide. Moreover, precisely evaluation of graphene layer coated silicon microring resonator (SMR) is crucial for future optoelectronic devices with compact footprint and low power consumption.

Multifractal characteristics of optical turbulence measured through a single beam holographic process.

We have previously shown that azopolymer thin films exposed to coherent light that has travelled through a turbulent medium produces a surface relief grating containing information about the intensity of the turbulence; for instance, a relation between the refractive index structure constant C(n)2 as a function of the surface parameters was obtained. In this work, we show that these films capture much more information about the turbulence dynamics. Multifractal detrended fluctuation and fractal dimension analysis from images of the surface roughness produced by the light on the azopolymer reveals scaling properties related to those of the optical turbulence.

Self-reconstructing all-optical poling in polymer fibers.

Self-sustained all-optical poling second-harmonic generation (SHG) experiments are conducted in single-core and multicore dye-doped poly(methyl methacrylate) optical fibers. By tuning the polarization of the fundamental beam, the SHG signal is degraded and is reconstructed spontaneously up to its initial level. We found a new situation in which the photo-induced self-organization of azo polymers creates a well-ordered periodic structure.

Simple turbulence measurements with azopolymer thin films.

A simple method to measure the influence on the laser beam propagation by a turbid medium is proposed. This measurement is based on the inscription of a surface relief grating (SRG) on an azopolymer thin film. The grating obtained with a single laser beam after propagation into a turbulent medium is perturbed and directly analyzed by a CCD camera through its diffraction pattern. Later, by scanning the surface pattern with an atomic force microscope, the inscribed SRG is analyzed with the Radon transform. This method has the advantage of using a single beam to remotely inscribe a grating detecting perturbations during the beam path. A method to evaluate the refractive index constant structure is developed.

Stable frequency doubling by all-optical poling in dye-doped polymer optical fibers.

We present experimental studies of all-optical poling (AOP) in large-core dye-doped poly(methyl methacrylate) plastic optical fibers. We show that the confinement of light within the fiber core permits us to reach the upper limit of second harmonic generation achievable by AOP using repeated trans-cis isomerization. Quantification of the output power indicates self-seeding that can counteract relaxation of the orientation during readout of the induced nonlinearity.

Cognitive ability experiment with photosensitive organic molecular thin films.

We present an optical experiment which permits us to evaluate the information exchange necessary to self-induce cooperatively a well-organized pattern in a randomly activated molecular assembly. A low-power coherent beam carrying polarization and wavelength information is used to organize a surface relief grating on a photochromic polymer thin film which is photoactivated by a powerful incoherent beam. We demonstrate experimentally that less than 1% of the molecules possessing information cooperatively transmit it to the entire photoactivated polymer film.

Multifractality of laser beam spatial intensity in a turbulent medium.

We present the results of a laser beam passing through a turbulent medium. First we measure the geometric parameters and the spatial coherence of the beam as a function of wind velocities. A multifractal detrended fluctuation analysis algorithm is applied to determine the multifractal scaling behavior of the intensity patterns. The measurements are fitted with models used in the analysis of river runoff records. We show the surprising result that the multifractality decreases when the spatial coherence of the laser is decreased. Universal scaling properties could be applied to the spatial characterization of a laser propagating in a turbulent medium or random medium.

Nonlocal communication with photoinduced structures at the surface of a polymer film.

Nonlocal communication between two laser light beams is experimented in a photochromic polymer thin films. Information exchange between the beams is mediated by the self-induction of a surface relief pattern. The exchanged information is related to the pitch and orientation of the grating. Both are determined by the incident beam. The process can be applied to experiment on a new kind of logic gates.